I am using the same version of cs3 on two different macs. On one mac when I create a 300 dpi file and save it at 300dpi as a jpg or png or psd it stays 300dpi. On the other mac when I do the same operation with the same file copied from a disc, it saves it at 240dpi and I can't switch it. Why does the software let me contreol the image dpi on one system and not the other?

This behaviour seems peculiar and unexpected – what exactly do you do on the affected machine?

Anyway it’s not dpi but ppi.

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Are Photoshop and OS fully updated and have you performed the usual trouble-shooting routines (trashing prefs by pressing command-alt-shift/ctrl-alt-shift while starting Photoshop until the appropriate dialog appears after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved and making a note of the Preferences you’ve changed, 3rd party plug-ins deactivation, system maintenance, cleaning caches, font validation, etc.)